FINISHING ALUMINUM

The aluminum surface can be finished in more different ways than any other metal. Thenormal finishing operations fall into four categories: mechanical, chemical, electrochemical,and applied. They are usually performed in the order listed, although one or more of theprocesses can be eliminated depending upon the final effect desired.

Mechanical Finishes

This is an important starting point since even with subsequent finishing operations a rough,smooth, or textured surface may be retained and observed. For many applications, the asfabricated finish may be good enough. Or this surface can be changed by grinding, polishing,buffing, abrasive or shot blasting, tumbling or burnishing, or even hammering for special effects. Rolled surfaces of sheet or foil can be made highly specular by use of polishing rolls; one side or two sides bright and textures can be obtained by using textured rolls.

Chemical Finishes

A chemical finish is often applied after the mechanical finish. The most widely used chemical finishes include caustic etching for a matte finish, design etching, chemical brightening, conversion coatings, and immersion coatings. Conversion coatings chemically convert the natural oxide coating on aluminum to a chromate, a phosphate, or a combination chromate– phosphate coating, and they are principally used and are the recommended ways to prepare the aluminum surface for painting.

Electrochemical Finishes

These include electrobrightening for maximum specularity, electroplating of another metal such as nickel or chromium for hardness and wear resistance, and, most importantly, anodizing. Anodizing is an electrochemical process whereby the natural oxide layer is increased in thickness over a thousand times and made more dense for increased resistance to corrosion and abrasion resistance. The anodic oxide forms by the growth of cells, each cell containing a central pore. The pores are sealed by immersing the metal into very hot or boiling water.

Sealing is an important step and will affect the appearance and properties of the anodized coating. There are several varieties of anodized coatings.

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J. G. Kaufman

Kaufman Associates

Columbus, Ohio

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz

Copyright  2006 by John Wiley & Sons, Inc.

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